LIMITS
- URL lifetime:  4 hours
- Dir lifetime: sliding, default 7 days (ttl= override, clamped [4h, 7d]);
  each add/edit/delete slides expires_at forward, capped at 30 days total
- Max file size: 5 MB
- Pool size:     100 MB (oldest files evicted first)
- Rate limit:    100 requests/min per IP
- Dir history:   last 50 entries kept per dir

PERSISTENCE — how long something lives, per type (also in each response's
"persistence" block):
- single file:  fixed 4h, not extendable (extendable_by:none)
- dir:          sliding lifetime (default 7d), extendable by activity
                (extendable_by:activity), capped at 30 days total
- bundle:       fixed 4h snapshot, not extendable (extendable_by:none)